(1) Field of the Invention
This invention relates to the step of drying carried out after the step of development of an electrophotosensitive member, for example, a microfilm with a liquid developer, and more particularly to a drying method which can attain a speedup of the drying step without causing non-uniform drying of the developed area of the electrophotosensitive member.
(2) Description of the Prior Art
In an electrophotographic recording (or copying) system of the so-called wet type in which an electrophotosensitive member, for example, a microfilm is developed with a liquid developer, picture image data are generally recorded or copied on the electrophotosensitive member such as a microfilm after being subjected to five processing steps, that is, the steps of charging, exposing, developing, drying and fixing. More precisely, in the first or charging step, the surface of the electrophotosensitive member is electrostatically charged as by corona discharge. In the succeeding exposing step, a picture image data to be recorded is projected onto the charged area of the electrophotosensitive member through a lens or the like to form an electrostatic latent image on the exposed area of the electrophotosensitive member. Then, the electrophotosensitive member having the electrostatic latent image formed thereon is fed to the developing step where a liquid developer is applied to the specific area of the sensitive surface, whereby the toner is attracted and held by the Coulomb's force on the specific area according to the pattern of the electrostatic latent image. Subsequently, the electrophotosensitive member is fed to the drying step where the solvent in the liquid developer is vaporized to be removed. In the final fixing step, the toner electrically held on the specific area of the sensitive surface of the electrophotosensitive member is fixed by means such as heating, pressurizing or resin coating. Thus, the picture image data is permanently preserved on the surface of the electrophotosensitive member.
The electrophotographic recording or copying system described above is advantageous in that, since all the processing steps including the steps of developing and fixing proceed sequentially after exposure of a picture image data, the recorded picture image can immediately be reproduced when so desired. Further, since the electrophotosensitive member is not sensitive to radiation until it is charged, the electrophotographic recording or copying system is advantageous in that the electrophotosensitive member need not be completely shielded against light during storage or during projection with a reader. Thus, recording by the electrophotographic method is now being put into practical use not only in the field of the so-called copier system but also in the field of the microfilm system.
In the electrophotographic recording system described above, the toner electrically attracted and held on the electrophotosensitive member prior to the step of fixing must be fixed immediately after the steps of exposing and developing. Further, when the electrophotosensitive member is in the form of, for example, a roll film or a microfiche film which is used for recording of many picture image data on many frames, the individual frames must be sequentially subjected to the processing steps described above. One of requirements in such a case is that the liquid developer must not flow out to a place other than the electrophotosensitive member or the liquid developer must not leak toward a frame or frames other than the frame to be developed. It is another requirement that the solvent in the liquid developer must be sufficiently vaporized to be removed from the surface of the specific area of the sensitive surface before the step of fixing. This is because, when the step of fixing is carried out while the solvent remains still on the surface of the specific area, the quality of the fixed picture image data will be degraded due to the tendency of occurrence of the blister on the specific area in the case of fixing by heating, the tendency of flowing of the toner in the case of fixing by pressurizing, or the tendency of uncuring of the resin in the case of fixing by resin coating. Therefore, a relatively large length of time has been required for the removal of the solvent.
In an effort to solve such a problem, a developing device as shown in FIG. 1 has been proposed. The developing device shown in FIG. 1 is commonly called a developing head when its developing section is formed independently of other processing sections, while the device is commonly called a developing section when it is formed integrally with other processing sections.
Referring to FIG. 1, the developing head or developing section is generally designated by the reference numeral 1 and has a cavity 12 which defines a developing chamber. A developing mask 2 surrounds or masks the periphery of the developing chamber 12, and an electrophotosensitive member 3 is brought into intimate engagement at its sinsitive surface with the end face 2b of the masking frame portion 2a of the developing mask 2 by a pressing plate or the like (not shown). The inner peripheral shape of the masking frame portion 2a of the developing mask 2 conforms to the shape of a predetermined area to be developed, for example, the area of one frame of the electrophotosensitive member 3. The developing head or developing section 1 is formed with three passages communicating with its developing chamber 12, that is, a liquid-developer introduction passage 5, a gas intake passage 6 and a discharge passage 7. A liquid developer 4 contained in a liquid developer tank 8 is supplied to the liquid-developer introduction passage 5 by a developer feed pump 13, so that the liquid developer 4 of a predetermined quantity is sprayed onto an exposed area of the electrophotosensitive member 3. A developing electrode 11 is disposed opposite to the sensitive surface of the electrophotosensitive member 3 to assist in the electrical attraction of the toner in the liquid developer 4 toward and onto the sensitive surface of the electrophotosensitive member 3. A portion of the liquid developer 4 sprayed onto the electrophotosensitive member 3 flows down along the sensitive surface of the electrophotosensitive member 3 to return through the discharge passage 7 into the developer tank 8. After the supply of the liquid developer 4 has been completed, that is, after the development has been completed, the excess of the liquid developer is squeezed. For this purpose, a valve 16 is interposed in the gas intake passage 6 connected to a gas source such as a gas pump or a gas cylinder (not shown). The valve 16 is opened to permit flowing of a gas such as air into the developing chamber 12 by way of the gas intake passage 6, so that the solvent of the liquid developer 4 remaining on the sensitive surface of the electrophotosensitive member 3 can be vaporized and the liquid developer 4 attaching to various portions including the inner wall of the developing chamber 12 can be blown away. At the same time, a suction pump 15 is operated so that the liquid developer 4 held in the gap between the electrophotosensitive member 3 and the end face 2b of the masking frame portion 2a of the developing mask 2 can be sucked to be trapped in the trap 14 through a suction slit 9 and a suction conduit 10. This sucking operation is continued until the specific area of the electrophotosensitive member 3 is fed to the succeeding drying step so that the substantial portion of the liquid developer 4 held between the end face 2b of the masking frame portion 2a of the developing mask 2 and the electrophotosensitive member 3 can be drawn out when the specific area of the electrophotosensitive member 3 leaves the end face 2b of the masking frame portion 2a.
The succeeding step of drying can be completed within a short period of time when the excess of the liquid developer 4 is squeezed in the manner described above while the specific area of the electrophotosensitive member 3 remains still in the position opposite to the developing chamber 12.
It has been considered preferable that the liquid developer 4 is to be squeezed as completely as possible in the developing step. FIG. 2 is a graph showing the results of an experiment conducted to find the relation between the duration of gas supply through the gas intake passage 6 and the quantity of the liquid developer 4 remaining on the electrophotosensitive member 3. A 16-mm microfilm was used as the electrophotosensitive member 3 in the experiment, and the area of the square opening of the developing chamber 12 was 10 mm.times.10 mm. In FIG. 2, the curve A represents the relation when squeezing gas only was supplied in the absence of any suction, while, the curve B represents the relation when squeezing gas was supplied in the presence of suction.
It will be seen in FIG. 2 that the quantity of the liquid developer 4 remaining on the sensitive surface of the electrophotosensitive member 3 shows a remarkable decrease within a period of time of 1 second after the starting of the gas supply only or after the starting both the gas supply and the suction, but it shows a very slight decrease thereafter. This is considered to be attributable to the fact that the liquid developer held between the electrophotosensitive member 3 and the end face 2b of the masking frame portion 2a of the developing mask 2 is not completely removed regardless of whether or not the suction is applied.
When, on the contrary, squeezing gas is continuously supplied for a long period of time of, for example, more than 2 seconds, partial drying starts to occur on the portion of the electrophotosensitive member 3 facing the opening of the developing chamber 12. Transfer of the electrophotosensitive member 3 in such a partly dried condition to the succeeding step of drying is undesirable in that, when the specific portion of the electrophotosensitive member 3 leaves the end face 2b of the masking frame portion 2a of the developing mask 2, the liquid developer 4 held therebetween flows out onto the dried surface portion of the electrophotosensitive member 3 and is dried again thereafter, thereby giving rise to non-uniform drying resulting in an objectionable degradation of the quality of the recorded picture image data.